Linear beam raster magnet driver based on H-bridge technique
An improved raster magnet driver for a linear particle beam is based on an H-bridge technique. Four branches of power HEXFETs form a two-by-two switch. Switching the HEXFETs in a predetermined order and at the right frequency produces a triangular current waveform. An H-bridge controller controls switching sequence and timing. The magnetic field of the coil follows the shape of the waveform and thus steers the beam using a triangular rather than a sinusoidal waveform. The system produces a raster pattern having a highly uniform raster density distribution, eliminates target heating from non-uniform raster density distributions, and produces higher levels of beam current.
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1) The United States of America may have certain rights to this invention under Management and Operating contract No. DE-AC05-84ER40150 from the Department of Energy.
FIELD OF THE INVENTION2) The present invention relates to a system for generating a uniform raster density distribution on a cryogenic target in order to eliminate beam-heating effects.
BACKGROUND OF THE INVENTION3) Lissajous raster systems used in linear accelerators typically include a resonance driver, which is operating in a high Q resonance loop. The resonance driver typically powers an air-core raster magnet with a sinusoidal current waveform. As the sinusoidal waveform approaches its peak, it slows down at the edge of the scan region in order to reverse direction. At the edges of the scan region, the scanning velocity of the electron beam becomes nearly zero. The slower scanning velocity causes much more beam energy to be deposited along the boundaries and the four corners as shown in the raster density 2D and 3D histograms of
4) Eventually, as a result of the increase in deposited energy in the boundaries and four corners, overheating occurs in the target material. Experimental measurements, determined by a luminosity scan along with a magnetic spectrometer, show that the luminosity decreases gradually as a result of the increase in beam current. This indicates that a local overheating effect near the boundaries and the corners of a Lissajous raster pattern contributes an uncertainty in the target length, which leads to a negative effect on the accuracy of the experimental data.
5) With the use of the prior art Lissajous raster system as described above, employing a magnet driven by a sinusoidal current waveform, the maximum allowable beam current is limited to about 200 μA to avoid overheating of the target.
6) What is needed is a system for producing a raster pattern for a linear beam having a highly uniform raster density distribution, elimination of target heating by non-uniform raster density distributions, and higher achievable levels of beam current. What is especially desired is a linear beam raster magnet driver that is capable of producing at least 100 A of linear current swing at 25 kHz for use with high-energy accelerator facilities and in applications such as medical therapy by heavy ion, cancer treatment by electron accelerators, ion implantation for semiconductor chip production, and modification of material behavior in material science.
SUMMARY OF THE INVENTION7) The present invention is an improved raster magnet driver for a linear beam. The linear beam raster magnet driver is based on an H-bridge technique. Four branches, each of which include a power HEXFET, form a two-by-two switch. Switching the HEXFETs in a predetermined order and at the right frequency produces a triangular current waveform. An H-bridge controller controls the switching sequence and timing. The magnetic field of the coil follows the shape of the waveform and thus steers the beam using a triangular rather than a sinusoidal waveform. The system produces a raster pattern having a highly uniform raster density distribution, eliminates target heating from non-uniform raster density distributions, and produces higher levels of beam current.
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16) The following is a listing of part numbers used in the drawings along with a brief description:
Description of the Present State of the Art:
17) Lissajous raster systems are typically used in linear accelerators to generate a raster density upon a cryogenic target. A critical component in the system is the resonance driver, which is operating in a high Q resonance loop. In the present state of the art, the resonance driver powers an air-core raster magnet with a sinusoidal waveform. As the sinusoidal waveform approaches its peak, it slows down in order to reverse direction at the edge of the scan region. At the edge of the scan region, the scanning velocity of the electron beam becomes nearly zero. This causes much more energy to be deposited along the boundaries and the four corners as shown in the 2D density histogram of
18) The large increase in deposited energy along the boundaries and corners regions eventually causes an undesirable overheating of the target material. Experimental measurements, including a luminosity scan with a magnetic spectrometer, shows the luminosity decreases gradually by the increase of the beam current. This indicates that a local overheating effect near the boundaries and the corners of the Lissajous raster pattern contributes an uncertainty in the target length that, in turn, affects the accuracy of the experimental data.
19) The prior art Lissajous raster system, of which the density histograms are shown in
Description of the Current Invention:
20) The present invention is a linear beam raster magnet driver based on an H-bridge technique. With reference to
21) As shown in
22) Based on the key parameters of the linear beam magnet driver as described above, the deposit beam energy in target material is uniformly distributed over the entire raster area without any enhancement at certain regions. The linear beam scan velocities in the two directions, x and y, are kept as high as possible to ensure the scanning beam travels the largest area at unit time in order to eliminate the local heating effectively.
23) The H-bridge 20, as shown in
24) Referring to the assembly diagram of
25) A phase lock (PLL) technique was used with the H-bridge controller 30. It has a large tolerance for any sudden changes in operational conditions. As an example, as the external trigger frequency disappears, the controller turns to the internal crystal oscillator yy automatically and smoothly with a response time of about 10 ms. Similar automatic functions are also established for power failure and other interruptions to give the driver protection against any external interruption.
26) A special raster frequency ratio of 1.00481, determined by a series of experimental observations, is applied to secure the best stability and uniformity of the raster pattern. This allows the two drivers, x and y, to operate at the highest frequencies.
27) The highly uniform density distribution of the beam scanning (uniform irradiation) in this invention has potential applications in fields other than high energy accelerators, including medical therapy by heavy ion, electron accelerators for cancer treatment, ion implantation for semiconductor chip production, and modification of material behavior in material science.
28) As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.
Claims
1. A system for producing a highly homogenous raster density distribution having at least 98% linearity and at least 95% uniformity at a high energy level of at least 100 A of linear current swing at 25 kHz comprising:
- an H-bridge circuit including a center portion and four ends;
- four branches of power HEXFETs forming a two by two switch;
- a raster air-core magnet at said center portion;
- a high voltage power supply connected at two of said four ends;
- an H-bridge controller connected to said HEXFETs;
- electrical pathways connecting said HEXFETs, said raster air-core magnet, said high voltage supply, and said H-bridge controller; and
- said H-bridge controller capable of switching said HEXFETs in an order and at a frequency to produce a triangular current waveform having a constant linear sweep velocity and a minimum turning time at the waveform peak.
2. The system of claim 1 wherein said constant linear sweep velocity is 1000 m/s.
3. The system of claim 2 wherein said minimum turning time at the waveform peak is 50 ns.
Type: Grant
Filed: Sep 20, 2004
Date of Patent: Jun 6, 2006
Assignee: Southeastern Univ. Research Assn. (Newport News, VA)
Inventors: Nikolai I. Sinkine (Newport News, VA), Chen Yan (Yorktown, VA), Cornelis Apeldoorn (Gloucester, VA), Jeffrey Glenn Dail (Poquoson, VA), Randolph Frank Wojcik (Yorktown, VA), William Gunning (Newport News, VA)
Primary Examiner: Tuyet Thi Vo
Application Number: 10/945,408
International Classification: G01V 3/00 (20060101); H05B 37/02 (20060101);